1. Field of the Invention
The present invention relates to an inverter apparatus and particularly to an inverter apparatus appropriated for correcting waveform distortions of the output voltage of an inverter controlled generator or the like.
2. Description of the Related Art
An inverter apparatus has commonly been employed in a portable or mobile power source, such as an engine-driven alternating current generator, for stabilizing the output voltage. It is known in a power source such as an engine generator that the output voltage is stabilized through feed-back controlling. However, such feed-back controlling of the output voltage is imperfect to produce a desired shape of its sine waveform. Preferable is feed-back of the waveform output. An inverter apparatus developed by, the applicants of this invention, is disclosed in Japanese Patent 2688660. The inverter apparatus is operated by pulse width modulation (PWM) technique and has a pulse width modulator circuit arranged to receive a switching output voltage waveform fed back from the inverter and release its resultant PWM signal.
The inverter apparatus disclosed in the above publication also has an output waveform feedback controlling means provided in the form of-an analog circuit. The analog circuit is however unfavorable because its components are increased in the number and have uneven performance thus changing the output voltage. Such uneven performance have to be significantly reduced. For the purpose, the analog circuit becomes intricate in the arrangement.
The analog circuit may be replaced with a microcomputer (CPU) which has a greater degree of data processability and can carry out a feedback control action at every PWM period. The CPU receives voltage waveforms or current waveforms of digital form which have been A/D converted from analog forms. It is however common that the resolution of every A/D converter is lower than the number of bits to be processed by the CPU. For example, the resolution of applicable A/D converters is as low as 10 bits while a standard CPU can handle 16 or 32 bits at once.
It is hence an object of the present invention to provide an inverter apparatus capable of feedback controlling with the best use of its CPU capability through improving the substantial resolution of an A/D converter.
This invention has the first feature, where an inverter apparatus having a direct current source circuit for converting an alternating current output of a generator into a direct current, a switching device for switching to control the direct current, a modulating means for pulse width modulating a reference sine wave signal at a predetermined frequency to produce a PWM signal, and a switching control circuit responsive to the PWM signal for driving the switching device to provide the alternating current through a pair of output lines, said inverter apparatus comprising a correction signal generating means for generating a correction of the reference sine wave signal in view of the alternating current waveform to correct the alternating current waveform on the output lines to a desired shape of the sine waveform; and an A/D converter having a plurality of input channels, wherein the alternating current waveform is received by at least two of the input channels of the A/D converter, and two channels of the alternating current waveform converted to their digital form by the A/D converter are summed and then transferred to the correction signal generating means.
According to the first feature, the alternating current waveform is received by a plurality of the input channels of the A/D converter. More specifically, the different signals are A/D converted at intervals of a time in a single input sampling action and are supplied to the correction signal generating means. As a result, the number of sampled data is increased and the A/D conversion can thus be improved in the resolution.
This invention has the second feature where said inverter apparatus also having a plurality of the A/D converters which can operate separately from each other, the A/D converters are operated at different timings lagged from each other for sampling the input channels, and the correction signal generating means receives a sum of the alternating current waveform converted into their digital form by the corresponding A/D converters.
According to the second feature, the sampling action of one of the A/D converters is commenced with time lag from sampling start of the other A/D converter, where the lag is shorter than the sampling period of one channel of the other A/D converter. As a result, a change in the signal during the minimum of the sampling period of the A/D converters can be identified with the use of a time lag of the sampling between the A/D converters.